In the drilling of an oil well it is desirable to learn as much as possible about the nature of the various formations penetrated by the well bore. To this end detailed records or logs of the formations encountered during the drilling of the well bore are kept. Such logs normally include observations and data from core samples as well as data generated by indirect means, including density logs, sonic logs and electric logs.
In electric logging, equipment which includes an electrical sensing device is moved through the bore hole and provides indications of the electrical resistivity or conductivity of the subsurface materials encountered at different depths. In resistivity logging variations in potential resulting from an electric current driven through the formation by the logging equipment are measured and recorded. The resistivity measurement thus responds primarily to conductive fluids contained in the formation.
Drilling and completion fluids very often are comprised of compositions which are not conducive to accurate electric logging. For example, drilling fluids may contain potassium chloride as a deterrent to the swelling of clay and the sloughing of formation materials into the fluid. High chloride ion levels in the drilling fluids interfere with the accurate interpretation of electric logs because they create a conductive medium in the fluid surrounding the logging equipment, thus hampering the investigation of the adjacent formations. Ideally, the logging apparatus should operate in a nonconductive medium so that there is a distinct resistivity contrast between the saline formation water and the logging medium.
As a means of overcoming the problem of electric logging in unsuitable drilling or completion fluids, compositions have been developed which are intended to be insensitive to salt water intrusion and to have a high tolerance for contamination from cement and other solids. These fluids, when used as drilling or completion fluids, are supposed to minimize the effect of ion-containing solutions in the formation.
Such fluids do not satisfactorily remedy the logging problem. They are relatively costly and, rather than creating an environment which does not contain the fluids or solids which interfere with accurate electric logging, they merely attempt to neutralize their effect. Further, they do not prevent the migration of crude oil into the well bore, which also tends to interfere with accurate data gathering.
It would be preferable to be able to eliminate unwanted fluids and solids from the well bore for an electric logging operation rather than to attempt to merely neutralize or counteract them. This would enable less expensive drilling and completion fluids to be utilized and would result in more accurate log data.